The invention relates to an ultrasound imaging system, and more particularly to an ultrasound imaging system having a reduced number of lines between the base unit and the probe. Ultrasound imaging systems are known, e.g., from "IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control", vol. 38, no. 2, March 1991, pp. 100-108.
In medical ultrasonic diagnosis, a volumetric area of the human body is irradiated with ultrasonic pulses and, from the reflected ultrasonic echo pulses, a signal processing unit constructs an ultrasound image which corresponds to a two-dimensional (2-D) section through the body. In known methods heretofore, to transmit and receive the ultrasonic pulses, linear arrays of piezoelectric transducer elements are generally used. The transducer elements are driven by an electronic control unit with preselected phase delays. Such phase-delayed linear arrays enable the transmitting and receiving of ultrasonic beams that can be steered and focused in a plane fixed from the normal to the array surface and the longitudinal direction of the array. The steering angle for the ultrasonic beam measured relative to the normal increases as the distance between the transducer elements becomes smaller. This interelement spacing is generally selected to approximately equal half of the wavelength of the ultrasound, to avoid additional diffraction patterns and, given a diagnostic frequency of 3.5 MHZ, amounts, for example, to about 0.2 mm. On the other hand, a certain minimum length of the linear array is necessary to achieve an adequate acoustic amplitude and an exact focusing of the beam. From these two requirements of the maximum distance of the transducer elements and the minimum length of the array, follows a minimum number of typically 64 transducer elements for the array.
Besides the known 2-D sectional images described above, three-dimensional (3-D) ultrasonic images are especially valuable for diagnostic medicine, because they enable dynamic structures, such as the flow of blood in the heart or in blood vessels to be depicted in real time. 3-D ultrasonic imaging systems are known having a 2-D transducer array, which is constructed as an N.times.M matrix from individual, generally square transducer elements. If these transducer elements are driven with properly selected phase delays, an ultrasonic beam can be produced and detected that can be steered and focused in two angular directions, in contrast to the one angular direction for linear arrays. To be able to scan a large enough solid-angle area with the ultrasonic beam, then analogously to the linear arrays, one must have a maximum interelement spacing of typically about 0.2 mm for the transducer elements and a minimum surface area of the 2-D array of typically about 20.times.20 mm.sup.2, in the case of a square array, i.e., N=M. Consequently, a minimum number of transducer elements is also required for the 2-D array. One should preferably strive for a number of about 100.times.100=10,000 transducer elements. Given such a large number of transducer elements and the required small dimensions, problems exist in the fabrication and contacting of the transducer elements and, in particular, by the number of control and data lines required to transmit the control signals and video signals.
Several 2-D ultrasonic transducer arrays for 3-D ultrasound imaging systems are known, which, however, are all composed of substantially fewer transducer elements, due at least in part to this problem.
In a known 2-D transducer array having 16.times.16=256 transducer elements, 96 elements are provided for transmitting and 32 elements for receiving ultrasound, and the remaining 128 transducer elements are not connected ("Ultrasonic Imaging", vol. 14, pp. 213-233, 1992, Academic Press).
Another known ultrasound imaging system contains a 2-D transducer array with 32.times.32 transducer elements in a hand-held instrument, to which are allocated 32 channels with 32 transmit pulsers for controlling the phase of the transducer elements and 32 channels with 32 pre-amplifiers for amplifying the received ultrasonic echoes. Via 32 transmit channel lines and 32 receive channel lines, the transmit pulsers or the pre-amplifiers are connected to a control unit or a signal processing unit. The transducer elements are interconnected into groups, which are intended to be used either only for transmitting or only for receiving. Only elements interconnected by groups in the rows, columns, or diagonals of the matrix can be controlled together or read out via the 32 transmit channels and the 32 receive channels. Thus, the ability to guide the ultrasonic beam through the solid angle is considerably curtailed ("IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control", vol. 38, no. 2, March 1991, pp. 100-108).